1. Field of the Invention
The invention relates to a system for injecting composite substances inside a forming mould.
2. Description of the Related Art
A composite moulding substance generally comprises a resin and appropriate reinforcing fibers or fillers. Patent Application FR-A-2 629 388 discloses an apparatus for injecting composite substance inside a forming mould, comprising one or more measuring devices, of the type incorporating a mobile piston, connected to a common supply device, likewise of the piston type, each measuring device being associated with an injection device mounted on the mould. This device is efficient for moulding composite substances comprising a thermosettable resin.
However, thermosettable resins have a relatively high density, of the order of 1.4 to 1.9, and generally do not make it possible to assemble the pieces made by a simple cooperation of shape, i.e. by clipping. Now, such a clipping may be desirable for bodywork or structural elements of an automobile vehicle, such as a radiator grill, a wing or an interior or exterior fitting. It has therefore been envisaged to use thermoplastics resins whose density is lower and which allow fixation of the pieces by elastic deformation.
One constraint of thermoplastics resins resides in the fact that they must be injected in the forming mould at a temperature of the order of 220 to 290xc2x0 C., the pressure of moulding being of the order of 300 to 350 bars. These values are substantially higher than the corresponding values for thermosettable resins, which are about 170xc2x0 C. and from 100 to 250 bars, respectively.
On the other hand, as a function of the geometry of the piece to be moulded, it is sometimes necessary that the injection device be installed on a mobile part of the mould in order that the mark possibly left on the finished piece at the point of injection be located in a concealed part of the piece once the latter is mounted. It is then necessary to provide a system of supply moulding substance to the injector which allows for movements of the latter due to the movements of the mould. It has been envisaged to install in the known device of FR-2 629 388 a flexible pipe between the measuring device and the injection device.
However, such a flexible pipe should be made with a reinforced pipe which is particularly expensive. Moreover, the geometry of the pipe would vary as a function of the position of the mobile part of the mould, with the result that the head losses induced by this pipe would be considerable and variable. Moreover, the known technology does not make it possible to use flexible pipes of large diameter at the pressures envisaged, with the result that the head losses induced in a flexible line are considerable. It is then necessary that the common supply device operate at very high pressure in order to attain, at the level of the impression of the mould, the necessary 300 to 350 bars when moulding a thermoplastics resin.
In addition, a flexible line necessarily comprises two connections allowing it to be connected at its two ends to a fluid circuit. Such connections necessarily present a reduction in the internal diameter with respect to that of the flexible pipe, this inducing additional, likewise inadmissible head losses.
Furthermore, one of the principal advantages of composite substances based on thermoplastics resins is that they may contain relatively long fibers, i.e. whose length may exceed 20 mm. Now, in a system provided with a flexible pipe, the long fibers risk being broken when passing the connections. Finally, whatever precautions are taken at the level of the supply or measuring devices, the temperature of the substance to be injected cannot be controlled when it transits in the flexible pipe, the latter being able to be several meters long, with the result that heat exchanges lead to the reduction in the temperature of the composite substance, which is detrimental to its fluidity and moulding properties.
U.S. Pat. No. 5,540,580 describes a system for injection of non-reinforced plastics material, provided with a supply line formed by articulated sections. The temperature control of the material in this line is provided by means of systems involving a small internal diameter of the line, hence considerable head losses.
The invention aims at solving these problems and at proposing a system of injection of composite substance capable of functioning with thermoplastics resins or thermosettable resins, including in the case of the injector being placed on a mobile part of the mould.
In this spirit, the invention relates to a system of injection of composite substance inside a forming mould, said system comprising a supply device equipped with a piston, at least one measuring device also equipped with a piston, and at least one line for supplying the mould with composite substance, characterized in that said line presents a substantially constant internal cross-section and is formed by articulated tube sections, said tubes being double-sheathed so as to allow the circulation of a heat-exchange fluid.
Thanks to the invention, the head losses induced in the line formed by articulated tube sections may be determined with great precision, particularly by calculation, and remain substantially constant whatever the position of this articulated line. In effect, the tubes conserve a substantially rectilinear constant geometry whatever their relative positions. Moreover, the circulation of a heat-exchange fluid makes it possible to maintain the composite substance injected at a temperature suitable for the moulding process, including when the articulated line has a length of several meters. Finally, the line formed by articulated tube sections does not comprise any connection with internal cross-section reduced with respect to the rest of the line, with the result that the fibers included in the substance to be injected do not risk being broken when passing such connections. The substantially constant internal cross-section of the line guarantees that no shear of the fibers takes place during the movements of the articulated line.
According to a first advantageous aspect of the invention, the line formed by articulated tube sections includes at least one rotating connection adapted to resist fluid pressures of the order of 350 bars. The use of a rotating connection makes it possible to obtain a degree of freedom necessary for the function of articulation of the line without having a negative influence on the head losses induced by the latter. In that case and according to another advantageous aspect of the invention, the rotating connection comprises a male sleeve penetrating in a female sleeve, these sleeves together defining an interstice provided with balls and each being associated with a bend. This construction guarantees that the rotating connection withstands the desired pressures and allows the articulation of the line of the system of the invention.
According to another advantageous aspect of the invention, the internal cross-section of the line formed by sections of tubes, is substantially constant. This makes it possible to maintain the head losses induced, at a minimum level.
According to a first embodiment of the invention, the line formed by tube sections is interposed between the measuring device and the mould. According to a second embodiment of the invention, the line formed by tube sections is interposed between the supply device and the measuring device. In that case, it may be provided that the measuring device be fixed on a mobile part of the mould. It thus forms with the injection device a compact assembly inside which the head losses and the variations in temperature may be optimized at their minimum values.
According to another advantageous aspect of the invention, the system comprises at least one injector mounted in the mould, this injector comprising a space for circulation of heat-exchange fluid up to the immediate proximity of the opening of the injector. This aspect of the invention makes it possible to control the temperature of the composite substance injected, up to its introduction in the mould.
According to another advantageous aspect of the invention, a thermal connection is placed around the injector in the mould. This aspect of the invention makes it possible to maintain the substance to be injected at a temperature different from that of the mould, up to its point of injection in the mould.
According to another advantageous aspect of the invention, the supply device and/or the measuring device and/or the injection device are double-sheathed so as to allow circulation of a heat-exchange fluid. This aspect of the invention thus makes it possible, in cooperation with the line formed by double-sheathed tubes, to control the temperature of the substance to be injected all along the injection line.